this would be my suggestion....the alpine type-r's hit HARD and sound amazing, they are my favority brand for a reason. Looking at this:
http://www.crutchfieldadvisor.com/learningcenter/car/subwoofers_wiring.html?subs=1&impedance=DVC4x2 you can get one 4-ohm DVE (dual voice coil) sub down to 2 ohms...you get a type-r alpine DVC 4-ohm (
http://www.onlinecarstereo.com/CarAudio/ProductDetail.aspx?ProductID=14954) and power it with this amp which is 600W at 2 ohms (
http://www.onlinecarstereo.com/CarAudio/ProductDetail.aspx?ProductID=14927) and the rule of thumb is you want a more powerful amp than your speakers. I know its only one sub, but one of these will hit reallly hard because of their quality and is the setup I would get (trunk space mostly) if I had the money. I can look for a 2 sub system in a little bit if you want but thats what I would recommend for now.